1. Field of the Invention
This invention relates generally to slide fasteners and more particularly to an automatic lock slider for slide fasteners which has a locking means capable of selectively locking the slider on the fastener elements against unintentional movement therealong.
2. Prior Art
A typical automatic lock slider of the type under consideration is well known in the art in which a locking spring member in the form of an elongated leaf spring or wire element is arranged longitudinally over the top or upper wing of the slider body. The locking spring member has one end anchored to the front end of the slider body and the other end bent inwardly to provide a locking pawl which is adapted to be introduced through an aperture formed in the top wing into the space between two adjacent fastener elements within the guide channel of the slider for locking the slider against movement along the fastener elements when the pull tab is laid flat against the top wing. The locking spring member also has a central humped or arched portion intermediate the opposite ends thereof. The pull tab has a transversely extending trunnion or pintle at one end thereof. The humped portion straddles and resiliently bears against the pull tab pintle to permit the pull tab to pivot thereabout between its flat and lifted positions. The pintle serves as a cam means which cooperates with the humped portion to move the locking pawl into and out of the space between adjacent fastener elements within the guide channel when the pull tab is actuated between its flat and lifted positions. The automatic lock slider has a drawback, however, in that the locking pawl oftentimes impinges upon one of the two adjacent fastener elements within the guide channel and fails to be smoothly introduced into the space between the fastener elements when the pull tab is actuated to its flat position, because the locking pawl moves between its locked and unlocked positions along a curved path oriented pitchwise of the fastener elements or in the longitudinal direction of the slide fastener. In such instance, the locking pawl is usually forcibly moved into the space between adjacent fastener elements within the guide channel by further pivotting the pull tab exactly to its flat position. This would result in accelerated wear of the locking pawl and give rise to premature malfunction of the automatic lock slider. Another disadvantage of the prior art slider is that when a severe pull tending to separate the opposed stringer tapes away from each other is exerted on the slide fastener with the slider held in its locked position, the locking pawl is forced toward the front end of the slider body by the fastener element, against which the locking pawl lockingly abuts, to permit the bend of the locking spring member into which the locking pawl merges to be deformed or yielded outwardly away from the slider top wing to such an extent that the function of the locking spring member is frequently adversely affected. This difficulty arises out of the fact that since the locking pawl moves along the curved path oriented pitchwise of the fastener elements, the aperture in the top wing through which the locking pawl is introduced into the guide channel has to have a relatively large size or length in the longitudinal direction of the slide fastener. Further, since the locking spring member is made of a leaf spring or wire element of a length sufficient to withstand operating fatigue and to be durable over an extended period of time, the slider body must necessarily be made long enough to permit such a relatively long spring member to be mounted longitudinally over the top wing.